1. Field of the Invention
The present invention pertains to a charge pump power supply for generating bipolar output voltages greater in magnitude than a single unipolar input voltage.
2. Description of the Prior Art
Digital interface circuits, such as RS-232 transmitter/receiver units, typically require a bipolar supply voltage of .+-.10 or .+-.15 volts. These interface circuits typically utilize charge pump circuits to convert a 5 volt unipolar supply voltage into the required bipolar supply voltage.
FIG. 1 is a simplified schematic diagram of a charge pump circuit disclosed in U.S. Pat. No. 4,897,774. Charge pump circuit 50 has a first input terminal connected to a power supply 12, which provides a supply voltage Vcc, and a second input terminal connected to ground (GND). Charge pump circuit 50 includes two voltage transfer capacitors 10 and 24, two reservoir capacitors 22 and 34, an inverted output terminal 38 and a non-inverted output terminal 40, all of which being selectively interconnected through switches 14, 16, 18, 20, 23, 26, 30 and 32.
Operation of charge pump circuit 50 is divided into two phases. In a first phase, voltage source 12 is connected across transfer capacitor 10, and the voltage on reservoir capacitor 22 is applied to transfer capacitor 24. During a second phase, the voltage on the transfer capacitor 10 is transferred to the reservoir capacitor 22, and the voltage on transfer capacitor 24 is transferred to reservoir capacitor 32.
FIGS. 2 and 3 show effective circuits formed during the first and second phases of operation.
As shown in FIG. 2, during the first phase of operation, switches 14 and 16 are closed and switches 18 and 20 are opened, thereby charging transfer capacitor 10 using voltage source 12. In addition, switches 23 and 26 are closed and switches 30 and 32 are opened, thereby charging transfer capacitor 24 to the voltage level of reservoir capacitor 22.
As shown in FIG. 3, during the second phase, switches 14 and 16 are open and switches 18 and 20 are closed, thereby connecting voltage source 12 in series with transfer capacitor 10. The sum of these voltages is applied across reservoir capacitor 22, and provides a voltage of approximately +2 Vcc at non-inverted output terminal 40.
In addition, during the second phase, switches 26 and 23 are opened and switches 30 and 32 are closed. In this state, the voltage across transfer capacitor 24 is applied across reservoir capacitor 34, and the positive end of transfer capacitor 24 is connected to ground through switch 32. The negative end of transfer capacitor 24 is connected to the side of reservoir capacitor 34, which generates a negative voltage at inverted terminal 38. One of ordinary skill in the art understands that a small "sag", or voltage loss, occurs when the positive voltage of reservoir capacitor 22 is converted into the negative voltage at inverted output terminal 38. Therefore, assuming the voltage level at non-inverted terminal 40 is +2 Vcc, the magnitude of the voltage level at inverted terminal 38 will be somewhat less than 2 Vcc (that is, between 0 and -2 Vcc).
A problem associated with the above-mentioned charge pump circuit 50 is that, when a large load is connected to the non-inverted output terminal 40, the negative voltage level at the inverted output terminal 38 can drop below the minimum voltage level required by the interface circuit. As explained above, reservoir capacitor 22 is used to charge transfer capacitor 24, which in turn in used to generate the negative voltage at inverted output terminal 38. When the load connected to non-inverted output terminal 40 is small, the voltage at non-inverted output terminal 40 can approach the ideal value of +2 Vcc. However, when the load on non-inverted output terminal 40 increases, the voltage level at non-inverted output terminal 40 can fall significantly below +2 Vcc (although still above a required minimum voltage). This reduced voltage is then subjected to "sag" during the conversion to the inverted voltage applied to inverted output terminal 38. As a result, the magnitude of the voltage level at the inverted terminal 38 can fall below the required minimum voltage, even though the voltage level at the non-inverted terminal 40 is equal to or greater than the required minimum voltage. When this occurs, the interface circuit incorporating charge pump circuit 50 fails to operate properly.